A cyclic deep-red bis-squaraine molecular scaffold for improving random laser performance

Abstract

A H-type cyclic bis-squaraine (bis-SQ) dye with deep-red emission is developed in this report to show enhanced random laser behaviors. Compared with the monomeric common squaraine dye (SQ), the rationally designed bis-SQ having cyclic interconnected dimeric squaraine architecture not only demonstrates about 20-32 nm bathochromic-shifts in the absorption and emission maxima, but also reveals significantly higher (∼4 times) fluorescence quantum yield and much longer (2-fold) fluorescent lifetime, which can be ascribed to the inhibition of internal energy loss and partial restriction of the intramolecular rotation in bis-SQ. The stimulated emission depletion (STED) microscopy and transient absorption spectra (TAS) studies of bis-SQ also confirms that it has much lower saturated excitation power (Psat) and saturated stimulated emission depletion power (Psted) in the range of 700–775 nm than conventional SQ, due to the larger stimulated emission cross section. The laser threshold of bis-SQ is also found to be markedly lowered than SQ which is about ∼20.8 μJ/cm2 for bis-SQ in comparison with ∼148.0 μJ/cm2 for SQ. This reduced laser threshold is instrumental in expediently implementing the bis-SQ dye as a random laser gain material in the deep-red region. Moreover, bis-SQ reveals excellent photostability and thermal stability which is essential for its potential application as a promising deep-red random laser gain material in future.